The petrography and geochemistry of the Clericy Pluton: an ultrapotassic pyroxenite-syenite suite of late Archaean age from the Abitibi region, Quebec

Abstract

The Archaean Clericy Pluton is a small, bimodal suite of ultramafic and syenitic rocks located in the Abitibi region of Quebec. The syenite unit consists predominantly of alkali feldspar and (aegirine)-augite and contains numerous elongated mafic xenoliths. The ultramafic component is predominantly clinopyroxenite. Using the geochemical screen proposed by Foley et al. (1987) the mafic to intermediate members of the suite (53.8% < SiO 2 < 60.9%) are shown to possess an ultrapotassic character. Crystallization, mainly of clinopyroxene and alkali feldspar, explains the transition from ultramafic to syenitic compositions. Magma mixing between a minimum melt granitic liquid and syenite is required to explain the most evolved compositions. The Clericy rocks were probably emplaced in a small release zone which developed along one of a number of major transcurrent faults which formed as a consequence of oblique subduction at the end of the Archaean era.